1. Field of the Invention
The present invention relates to an illumination optical system, an illumination apparatus using this system, and an observation system using the illumination apparatus.
2. Description of the Related Art
As an illumination optical system for illuminating a broad area of an object, there is a system in which a light emitting device such as a lamp or an LED, or an end surface of a light guide fiber bundle that transmits light is used as a light source, and light emitted from the light source is diverged by a negative lens or a positive lens.
In the illumination optical system constructed using a negative lens, to secure a broad area on which illumination light is distributed with the negative lens, a loss is generated in the quantity of light, unless the outer diameter of the negative lens is increased. Therefore, for example, in an illumination optical system using a negative lens, it is difficult to broaden the light distribution area to such an extent that the light distribution angle exceeds 120°. When the light distribution area is broadened, the radius of curvature of the concave surface of the negative lens is reduced, and it becomes difficult to work the lens.
In an illumination optical system constructed using a positive lens, even when the outer diameter of the positive lens is small, the light distribution area can be broadened. However, to distribute the illumination light more broadly, the light refracting action of the positive lens has to be strengthened. Therefore, the radius of curvature of the positive lens decreases, and workability of the positive lens drops. It is difficult to realize an illumination optical system having light distribution characteristics such that the light distribution exceeds 150° when using a positive lens.
Moreover, when a light guide fiber bundle is used as a light source of the illumination optical system, there is a problem that light distribution unevenness is generated. This will be described with reference to FIGS. 32A, 32B and 32C.
A light guide fiber bundle 1 is formed by bundling a plurality of optical fibers. Each fiber is made up of a core and a cladding layer that surrounds the core; only the core part transmits the light. The cores of the fibers are arranged in a dot matrix form in an emission end surface of the light guide fiber bundle as shown in FIG. 32A. When an illumination optical system formed by a positive lens is used, as shown in FIG. 32B, the emission end surface of the light guide fiber bundle 1 is enlarged and projected on the object surface 4. Therefore, on the surface of the object, a portion corresponding to the core of the end surface of the light guide fiber bundle becomes brighter than other portions. As shown in FIG. 32C, the light distribution unevenness is generated in dot matrix form on the object surface 4.
Furthermore, since the light is separated into colors by dispersion of the glass material of the lens included in the illumination optical system, there is a problem that color unevenness is generated in the peripheral portion of the illuminated area. The color unevenness is generated in the case where the illumination optical system is composed of a positive lens. The unevenness will be described with reference to FIG. 33A. The glass material of the lens has dispersion properties (the refractive index varies dependent on the wavelength of light). Therefore, a light ray that exits from the end surface of the light guide fiber bundle, travels in parallel with the optical axis of the positive lens, and enters the positive lens, is given an angle of refraction which varies dependent on the wavelength of the light ray that is incident on the surface of the positive lens. As a result, the light ray that exits from the positive lens is separated into colors. Therefore, a color unevenness is generated. For example, the outside of an illuminated area becomes slightly bluish. Moreover, when the radius of curvature of the refractive surface of the positive lens is reduced, and the refractive power of the positive lens is strengthened in order to distribute the illumination light broadly, this unevenness becomes quite noticeable.
Laid-Open Japanese Patent Application No. 6-148519 discloses an illumination optical system for reducing generation of the above-described light distribution unevenness or color unevenness.
The Laid-Open Japanese Patent Application No. 6-148519 shows an illumination optical system used on the emission side of the light guide fiber bundle. The illumination optical system is formed of only a positive lens, or formed of a positive lens and a single fiber, the single fiber being disposed on the emission end surface of the light guide fiber bundle. The positive lens is a plano-convex lens whose convex surface is an aspherical surface.
In the illumination optical system of the Laid-Open Japanese Patent Application No. 6-148519, the convex surface of the positive lens is formed into an aspherical shape, and the light distribution area is broadened in such a manner that the color unevenness is generated outside the observation area, so that observation is not obstructed.
Further, in the Laid-Open Japanese Patent Application No. 6-148519, it is proposed that the lens surface of the illumination optical system be roughened in order to reduce illumination unevenness. That is, in the illumination optical system in which the convex surface of the positive lens is formed into a polished surface, as shown in FIG. 33A, a color unevenness is generated in the illumination light. However, when the convex surface of the positive lens is formed as a roughened surface as shown in FIG. 33B, the ray that enters the roughened surface is diffused in a random direction regardless of the wavelength, no color unevenness is generated, and the illuminated area can be set to uniform brightness. Moreover, the illuminated area is broadened by the random diffusion of light by the roughened surface, and wide angle illumination is achieved.
The Laid-Open Japanese Patent Application No. 2000-193894 describes a lens surface formed into a light diffusing surface, that is, a roughened surface formed by grinding. The ground lens surface is chemically treated with hydrogen fluoride. Accordingly, the loss of the quantity of light caused by the roughened surface is suppressed to raise transmittance. Furthermore, time for the chemical treatment is controlled, so that the loss of the quantity of the illumination light is balanced with the light distribution unevenness.
In the illumination optical system disclosed in the Laid-Open Japanese Patent Application No. 2000-193894, the plano-convex lens that forms the illumination optical system has a roughened surface on its plane side whose degree of roughness is set in such a manner that the loss of the quantity of light is about 10% of that of the lens finished into a polished surface. The light distribution property of this plano-convex lens is the same as that of the lens finished into a polished surface, and the roughness level of the roughened surface is very close to that of the polished surface.
Moreover, in the Laid-Open Japanese Patent Application No. 2000-193894, an example of the working of the roughened surface of the lens, in which the surface is ground by an abrasive wheel having a mesh size of about #800 is give. However, during the working of the lens, the surface state of the abrasive wheel changes. For example, the surface of the grindstone is clogged with residue or polishing waste generated at a time when abrasive grains on the surface of the abrasive wheel are abraded. Furthermore, the surface state of the lens surface to be worked changes with working conditions such as the number of revolutions of the abrasive wheel or the lens to be worked, and time. Since the roughened state of the lens surface cannot be fully controlled by the mesh size of the abrasive wheel for the working, the roughened state of the lens surface required for securing desired optical properties has been unknown.
Laid-Open Japanese Patent Application No. 2001-292956 discloses a negative or positive lens on which a roughened lens surface is formed. However, it is simply described that a lens having a roughened surface is used, and the roughness of the abrasive wheel for the working is simply described in the same manner as in the conventional technique. But the roughened state of the lens surface is not described.
Furthermore, in the conventional illumination optical system, even in the case where the above-described dot-matrix light distribution unevenness is inconspicuous, radial light distribution unevenness is sometimes generated in a peripheral portion of the illumination field. When the illumination unevenness is insufficiently reduced, light distribution unevenness is generated as shown in FIG. 34. In the example of FIG. 34, a plane is illuminated with the illumination light, and the central portion of the illumination field is interrupted and blackened so that the peripheral portion of the illumination field is easily seen.